Overview of refractive index sensors comprising photonic crystal fibers based on the surface plasmon resonance effect [Invited]

Fig. 1 Schematic diagram of photonic crystal fiber surface plasmon resonance sensing structure

Over the past few decades, optical fibers have been widely applied to telecommunication, imaging, lasers, and sensing. Optical fibers constitute a platform to excite surface plasmon resonance (SPR) when plasmonic materials are coated on the surface of the fiber core, thus satisfying the need for miniaturization and integration. In particular, photonic crystal fiber (PCF) sensors based on SPR (PCF-SPR) have drawn much attention due to the flexibility of their structure. A PCF is essentially a microstructured optical fiber consisting of a silica core surrounded by a periodic lattice of air holes along the length of the fiber. PCFs are regarded as a desirable platform to excite surface plasmon resonance (SPR) because of easy realization of phase matching conditions between the fundamental core mode and the plasmonic mode, which is the most unique advantage over conventional optical fiber SPR sensors.

In recent years, PCF-SPR sensors have become a hot research area in the optical sensing field, and various types of PCF-SPR sensors have been proposed and investigated theoretically to cater to a myriad of applications ranging from biology to chemistry monitoring. Compared with conventional single-mode fibers, PCF has become a flourishing research field for SPR because of the outstanding features such as single-mode operation over an extended range of operating wavelengths, large mode area as well as overall controllable dispersion. Based on the physical mechanisms that govern enhanced coupling between the core-guided mode and the plasmonic mode, most of the research efforts have focused on the following aspects: (1) broadening the RI detection range, (2) adjusting the resonance wavelength, and (3) increasing the sensing sensitivity.

The research group led by Prof. Chao Liu from Northeast Petroleum University summarized the refractive index sensors comprising PCF-SPR effect and the challenges and future perspectives are outlined. This overview paper is published in Chinese Optics Letters, Volume 19, No. 10 2021 (C. Liu, et al., Overview of refractive index sensors comprising photonic crystal fibers based on the surface plasmon resonance effect [Invited]).

The current status of PCF sensors based on SPR is summarized in this work. The physical principles governing SPRs are discussed, and the important simulation methods for PCF-SPR sensors are summarized. The aims of PCF-SPR simulation include the process of selecting the optimal design and using models to predict the sensing performance for each structural combination. Many plasmonic PCF-SPR structures have been simulated by the finite element method (FEM), which is the most common method to compute the optical properties of PCF-SPR sensors. Based on FEM, PCF-SPR sensors with various structures have been designed and evaluated numerically, for instance, side-polished, dual-core cladding with open rings, and dual-beam, liquid-filled cladding coated by plasmonic nanomaterials. Moreover, this review addresses the desirable attributes of PCF-SPR sensors including wide RI detection ranges, controllable resonance wavelengths, and high sensitivity. Different PCF-SPR sensors with unique structures and their sensing characteristics are also discussed. The fabrication and the comparison of performances are also illustrated, and the challenges and future perspectives are outlined. Fabrication of PCF-SPR sensors involves two aspects of manufacturing PCF and producing plasmonic nanomaterials. Advances pertaining to the manufacturing and post-processing technology of optical fibers have spurred substantial development in techniques such as extrusion, casting/molding, mechanical drilling, and stack-and-draw. PCF sensors based on the SPR effect will be used to detect temperature, strain, magnetic field, human IgG, and cell concentrations, exhibiting great application potentials.

Theoretical calculations show that performances of the sensors remarkably depend on parameters such as the air hole size, air hole pitch, plasmonic material thickness, and fiber core diameters. Although there has been tremendous progress in theoretical simulation and experimental fabrication of PCF-SPR sensors, not all aspects of PCF-SPR sensors are well understood. There are a few key barriers that are preventing more widespread implementation. The biggest obstacle is concerned with the fabrication of PCF-SPR sensors, especially deposition of high quality plasmonic materials on the air hole inner walls inside the PCFs. In addition, although plasmonic materials can be filled in the air hole channels in the PCFs by high-pressure chemical deposition, high temperature pressure injection, and pressure-assisted splicing techniques, these techniques are quite complex from the manufacturing perspective, and more efforts are needed to develop simple and reliable fabrication techniques for plasmonic materials.

The overview of refractive index sensors comprising PCF-SPR effect. Among the different structures, the D-shaped PCFs have large potential from the viewpoint of fabrication. The D-shaped PCF-SPR sensors possess the advantage of relatively easy side polishing, and plasmonic materials can be fabricated by physical deposition techniques such as RF/DC magnetron sputtering, electron beam evaporation, and chemical synthesis. The sensitive metal layer can be coated adjacent to the core of the PCFs to promote the interactions with the analyte and enhance the sensing performance. In addition, alternative plasmonic materials like graphene, MoS₂, and other 2D materials with unique physical properties are being explored and expected to improve PCF-SPR sensors in specific applications. The field of PCF-SPR sensing is vibrant, and a better fundamental understanding and continuous technological advances will expedite commercial development.

综述|基于表面等离子体共振效应的光子晶体光纤传感器

图1 光子晶体光纤表面等离子体共振传感结构示意图

东北石油大学刘超教授课题组在Chinese Optics Letters2021年第19卷第10期上发表综述论文(C. Liu, et al., Overview of refractive index sensors comprising photonic crystal fibers based on the surface plasmon resonance effect [Invited])，介绍了光子晶体光纤表面等离子体共振（SPR）传感器的研究现状，讨论了SPR效应的物理机制，总结了光子晶体光纤SPR传感器的理论模拟方法和典型研究成果，归纳了光子晶体光纤SPR折射率传感器研究所面临的挑战和未来发展的前景。

研究现状

表面等离子体共振(Surface plasmon resonance, SPR)是纳米金属薄膜表面自由电子与光子相互作用而产生的一种谐振吸收现象，是光子与电子微观相互作用在宏观能量上的体现。表面等离子体纳米材料与光纤的集成化构成的新型光纤SPR传感器，其设计、制作和应用涉及材料学、光学、化学等多个学科的交叉融合，是最为诱人的研究领域之一。

近年来，光子晶体光纤SPR传感器凭借其优异的传感性能在生物、化学、医学等诸多领域展现出巨大应用潜力，而成为国际光学领域前沿研究热点之一, 其典型结构如图1所示。与传统单模光纤相比，光子晶体光纤具有单模工作波长范围宽、模场面积大和色散整体可控性好等突出优点。最近，许多结构新颖、功能独特的光子晶体光纤SPR传感结构被相继提出，并开展了大量的基础理论研究工作，研究思路主要集中在如下三方面：(1)拓宽RI探测范围；(2)调节共振波长；(3)提高传感灵敏度。

模拟仿真研究

目前，有限元法是计算光子晶体光纤SPR传感器光学特性最常用的方法。利用该方法，科研人员对不同结构的光子晶体光纤SPR传感器进行了设计和数值计算，包括侧边抛磨型、包层双开口环型、双光纤束型、液体填充包层型，其目的是通过设计优化模型来预测传感器的性能。该综述从折射率检测范围、共振波长调控范围以及灵敏度三个视角，介绍了多种光子晶体光纤SPR传感器的理论模型及其传感特性，对其制备和性能进行了说明和比较，并分析了该领域所面临的机遇、挑战和发展前景。

挑战与突破

理论计算表明，光子晶体光纤SPR传感器的性能显著依赖于空气孔尺寸及其间距、等离子体纳米材料厚度和纤芯直径等参数。虽然光子晶体光纤SPR传感器在理论和实验方面取得了长足进展，但传感器的制作，特别是如何在光子晶体光纤空气孔内壁上沉积高质量的等离子体纳米材料成为制约其发展的最大瓶颈。

尽管可通过高压化学气相沉积、高温压力注入和压力辅助熔接技术将等离子体纳米材料填充到空气孔通道内，但从制造角度来看，这些方法工艺复杂、技术难度大。因此，急需研制和开发简单高效的等离子体材料制备方法和技术。

该文章综述了 光子晶体光纤SPR折射率传感器的研究现状，指出在众多的传感结构中，D型光子晶体光纤SPR传感器可通过侧边抛磨方法进行微加工，其制作工艺相对简单，更具应用潜力。另外，可通过物理沉积方法，如：射频/直流磁控溅射、电子束蒸发以及化学合成等方法将等离子体纳米薄膜沉积于D型光子晶体光纤表面。等离子体敏感层涂覆在光子晶体光纤纤芯附近，能够促进其与分析物间的相互作用，从而提高传感性能。

前景与展望

光子晶体光纤SPR传感器的加工制作涉及光纤制作和等离子体纳米材料制备两个方面。随着光子晶体光纤制造工艺、技术以及后续处理技术的迅猛发展，挤压、铸造/成型、机械钻孔和堆叠拉拔等技术将获得实质性发展。未来，光子晶体光纤SPR传感器有望在温度、应变、磁场、人体IgG和细胞浓度等领域展现出巨大应用前景。该综述指出，未来的研究可专注于石墨烯、MoS₂和其他具有独特物理性能二维等离子体材料与光子晶体光纤的结合。可以预见，光子晶体光纤SPR传感领域充满活力，深入的基础理论研究和技术进步将加速其实用化和商业化发展。